1. Field of the Invention
The present invention relates to a serial two-stage extruder for melting and mixing plastics, rubber or the like, and performing granulation and extruding various products such as films, sheets, and pipes.
2. Description of the Prior Art
A serial two-stage extruder in which two extruders are connected in series has been conventionally used to melt and mix raw material, especially in preparation for directly forming films, sheets or the like from the raw material. In fact there is no practical means to extrude a raw material that is difficult to melt and mix other than the known serial two-stage extruders. Therefore, this two-stage extruder is now employed especially frequently. Before this serial two-stage extruder was used, raw material, that was difficult to melt and mix, was first granulated (pelletized), and films, sheets or the like were manufactured in a separate shaping line by making use of the prepared pellets.
FIG. 21 is a schematic of a serial two-stage extruder in the prior art. In this figure, an extruder shown at the right upper section is a first-stage extruder, an extruder shown at the left lower section is a second-stage extruder, and an extruding end of the first-stage extruder and a molten resin introducing end of the second-stage extruder are connected by a connecting pipe 7.
Normally, a screw 1 and a cylinder 2 of the first-stage extruder have a small diameter and a high speed to carry out the plasticizing and melting of the raw material. A screw 8 and a cylinder 9 of the second-stage extruder have a large diameter and a low speed and carry out the cooling, homogenizing and extruding of the raw material. It is to be noted that in FIG. 21, reference numeral 3 designates a speed-reduction drive unit, numeral 4 designates a driving electric motor, numeral 5 designates a warm-up heater, numeral 6 designates a hopper, numeral 10 designates a speed-reduction drive unit, numeral 11 designates a driving electric motor, numeral 12 designates a warm-up heater, numeral 13 designates a cylinder cooling unit, numeral 14 designates a die, and numeral 15 designates an extruded product. The above-described type of serial two-stage extruder in the prior art is disclosed in U.S. Pat. No. 3,860,220.
If the necessary effects on the resin are respectively produced by two extruders as described above, it becomes possible to extrude a large amount of product with good quality by means of a compact extruder. Furthermore, in order to achieve these results with materials which are hard to mix, an extruder was proposed in Laid-Open Japanese Pat. Specification No. 3-126519 (1991) in which the barrel of the first-stage extruder has a polygonal cross section over part of or the entire longitudinal axis of the barrel.
Normally, the first-stage extruder assumes a role of melting and mixing the raw materials, while the second-stage extruder assumes a role of homogenizing the molten materials and stably extruding the molten materials against resistance offered by the die. However, recently, materials which are hard to mix have been used increasingly to manufacture diverse and complex products. That is, the use of polymer blends which are mixtures of different kinds of resins and the use of resin mixed with a large amount of various kinds of fillers have increased. In some cases, depending on the type of raw material, the screw of the first-stage extruder must be rather long to sufficiently mix the raw materials.
In the extruder disclosed in Laid-Open Japanese Patent Specification No. 3-126519 (1991), a barrel having a polygonal cross section offers some improvement in feeding and plasticizing raw materials depending upon a number of corners of the polygonal shape of the cross section and the lead and the like of the screw of the first-stage extruder. However, in practice, insufficient mixing may still occur with certain raw materials.
The screw of the second-stage extruder aims at homogenizing the resin almost melted in the first-stage extruder and at extruding the resin at a constant rate to stabilize both the quantity and temperature of the resin. Temperature differences of the resin within the screw groove, produced by a transverse flow of the resin passing through the screw of the second-stage extruder, are eliminated by a dulmage at the tip end of the screw as shown in FIGS. 22 to 25. However, the dulmage in the prior art effects limited mixing and kneading actions. Thus, if the screw groove is too deep, temperature difference of the resin produced within the screw groove cannot be eliminated. Hence, resin having a temperature distribution (temperature differences throughout the resin) was extruded, resulting in unacceptable products. Furthermore, it was impossible to enhance the function of the second-stage extruder to a desired extent. In this connection, the depth of the screw groove of the second-stage extruder in the prior art is typically 0.05 times as small as the outer diameter of the screw or larger, namely 0.1 times as small as the outer diameter or smaller.
FIG. 22 shows one example of a representative dulmage (4-stage). FIG. 23 shows a cross section of the screw taken through a stage of fins of the dulmage. In the screw of a uniaxial extruder shown in FIG. 22, the dulmage 132 is provided at the tip end of a screw 131 for effecting a mixing-kneading action. This dulmage 132 is formed of a number of stages 135a of fins 133 defining grooves 134 therebetween, and a cylinder portion 135 having a somewhat smaller outer diameter than a pitch circle of the grooves 134.
In FIG. 22, resin fed from the screw 131 to the dulmage 132 is divided by the fins 133, then flows downstream while turning within the small semicircular grooves 134, and is rejoined in the cylinder portion 135 to be mixed and kneaded. Subsequently, the resin is again divided by the downstream fins 133, and enters the corresponding grooves 134 so that mixing and kneading of the resin is repeated.
Here, attention will be paid to the turning of the resin within the grooves 134. FIG. 24 shows a state of the resin within a groove 134 in a simplified manner, in which state the resin 136 proximate the surface of the screw moves along a surface defining the bottom of the groove 134 as shown by an arrow 137. Thereafter, it is forced to the outer circumference of the screw along the surface defining the groove 134, and moves along the inner surface of the cylinder as shown by arrow 138. Thereafter, it again moves along the surface of the screw defining the groove 134 towards the bottom of the groove 134, and as shown by an arrow 139, it flows from the groove 134 to the downstream cylinder portion 135.
It is well known that the number of times the resin traverses a groove 134 in each stage of fins depends upon the operating conditions of the extruder, that is, the rotational speed of the screw, the cylinder temperature, the pressure of resin at the tip end of the screw and the like, provided that the shapes of the grooves 134 are identical.
Accordingly, in a uniaxial extruder having a dulmage as shown in FIG. 25, resin 140 existing at the bottom of the groove in the inlet portion of the groove 134 would traverse the surfaces defining the groove 134 and the cylinder 130 as indicated by arrows 141, 142 and 143, would then reach the bottom of the groove at the outlet of the groove 134, and in some cases, the phase of the resin within the fin grove would not vary. In such case, the dividing of the resin with the fins 133 and the causing of the resin to revolve within the grooves 134 is insignificant.
As described above, in the dulmage in the prior art shown in FIG. 22, the configurations of the fins 133 and the grooves 134 in each stage are the same. Hence, the number of revolutions of resin within the grooves 134 are equal. If the resin existing at the bottom of the groove in the inlet portion of the groove 134 makes one revolution or a whole number of revolutions within the groove 134, in same cases, resin fed from the screw 131, especially resin existing proximate the surface of the screw 131, almost would not be mixed and kneaded even after passing through the dulmage. Under such an operating condition in which the mixing and kneading performance is extremely poor, an uneven temperature distribution and an unevenly kneaded state would arise.
As described above, the heretofore known dulmage type of screw employed in a uniaxial extruder or an injection molding machine has a structural shortcoming in that there exists an operational range wherein the mixing and kneading of the resin is poor giving rise to an uneven temperatures distribution and an unevenly kneaded state of the resin.